Concrete strength - it is hard to fail!

posted Jan 13, 2018, 4:49 PM by jeffery jim

Many resident engineers for civil and structural engineering will jump up when G30 concrete did not reach fck,cb= 30N/mm² on the 28th day. Should we be worry if this concrete didn't achieved the targeted strength? No? I say, it is hard for concrete strength to fail to meet the minimal requirement(s) for a characteristic strength.

1. Usually, i am not worry about concrete strength that didn't achieved required strength. Similar to court case, it is harder to prove that the concrete beyond reasonable doubt is not a giving the required characteristic strength.

2. When 7th day concrete strength does not hit the two-third of the required strength, the first thing to do is to identify the cement type. Now, when design mix was done, most of the mix will use ordinary portland cement which is classified as CEM I where it is indicative to use this rule of thumb to justify the strength development. In certain cases, when batching plant is running out of CEM I, there is likelihood to replace it with other type or class of cement. Apart from using replacement cement, certain construction may not use CEM I class cement. For example, for the construction of large raft foundation or pad footing, there is likelihood to use ggbs cement or portland-slag cement which is in either class CEM II/A-S or II/B-S in order to control thermal crack from affecting the large volume of concrete due to hydration in large volume. This class of cement is usually slow in developing their strength and go up to 60 days to achieve their full strength and continue to develop their strength through time and at time may reach 110% or 120% of their prescribed capacity. Therefore, it is a good practice to have high sampling rate when starting with work involving large mass concreting. Sadly, the sampling frequency in code of practice is against this where it goes by volume, where my suggestion is based on experience and practicality when it is critical.

2a. Apart from the earlier mentioned issues from slow strength development, the recommended high sampling rate as i reckoned is based on my experience involving failure of concrete to achieve characteristic strength and the failure to provide data for statistical analysis since the batching plant was erected on site recently and have no data in regards to concrete batching quality. Further explanation will be in para 3.

3. There are few ways to justify it before taking into consideration to remove it. The first way is to conduct statistical analysis and/or to conduct core test of in-situ strength through concrete core. There are two criteria to be fulfilled in order to get a pass for concrete strength in this case. First, it is considered as individual test result and second, group test result. For statistical analysis, the standard deviation shall be calculated from at least past 35 consecutive batching and test result over a period of at least 3 months. For that reason, statistical analysis is always a problem for newly assembled batching plant at site. The result of last 15 test should not deviate from 0.63σ <s15 <1.37σ. Remember, what we are looking is a standard normal distribution where s15 value is on one-tail, not two-tail of the distribution. What it means is that s15 should be higher than the mean value of the target/desired strength. At σ=0.63, 74% of the concrete achieved the targeted strength and at σ=1.37, 91% of the concrete achieved the targeted strength. Previously, the minimal requirement of in-situ characteristic strength to designed characteristic strength was 90%, now reduced to 85%.

3a. There are a few requirements in code of practices that require fcu to be as high as 90% of the targeted strength. When we calculate shear stress in beam, the required ν value should not exceed 0.8 or 80% of fcu or 5N/mm². Same goes with the value for punching shear when designing pile cap. This mean, the contingency available before breaching the design is 10% and allowance of 6% to breach the design.

4. When statistical analysis is not helping at all, the next phase should involve core extraction. In this case, you have two options in order to compute your in-situ characteristics strength. There are two approaches for this where Approach A with at least 15 cores and Approach B with 3 to 14 cores. Approach B is cost saving but it will definitely provide lower reading where it mean of cores sampled minus 5N/mm² to 7N/mm². Approach A somehow allows mean of cores sampled minus 2.96N/mm². Nevertheless, both approaches generously allow in-situ characteristic strength to be determined as +4N/mm² from the lowest reading. In this case it is usually a pass unless you ordered the wrong concrete grade.

5. What if in an extreme case where both methods failed? Should we start to demolish and reconstruct? It depends on how adequate your SO or the (consulting) engineers in understanding the whole code of practice rather than to subscribe to only Part 1 of the BS8110. Under Clause 7.2 Part 2 of the mentioned code of practice, there is an allowance for increase of strength of concrete based on modulus of elasticity. However, this assumption was eliminated in Part 1 since the assumption of no increase of strength is part of satisfying the limit state requirements. That is the most one can do to justify if this incident occurred for super structures.

6. Foundation and bases come with much loopholes. Most of the foundation designs in Malaysia will use G35 concrete. Is this the actual requirement when designing? Often people will say, the reason behind the use of G35 is for strength of the foundation. No! That is a false claim and you can refute and rebuke such need. Justification can be made based on para 6a.

6a. This need derived from the much up to date BS8110-1:1997 then which mentioned clearly that the grade of concrete should take account of fcu for limit state design and also durability in clause 3.1.7.2. The cross-reference of durability is made to BS5328:1997. Part 2 of BS5328 in Table 6 suggest the use of G35 for FND2 class. When BS5328 is repealed and BS8500 (read in the light of BS EN 206) is in place, the FND2 class concrete minimum grade is reduced to G30 as in Table A.14 regardless if cement class CEM I is in use. All FND class can be as low as G30 concrete. Remarks: BS8110 has undergone 2 revisions since 1985 and the big bulk of such changes is due to durability issues.

6b. What does this mean? For almost 9 years (1997 to 2006), concrete society found out that the use of G35 is not necessary. The reason behind the selection of this grade for durability purposes is to allow the low cement/water ratio to provide denser concrete and less voids. This is replicating the need of blinding materials around the concrete.

7. To summarize this write-up, it is harder to reject concrete samples which are below the design characteristic strength than to accept it. This is a simple guideline when reviewing and planning for justification when concrete strength failed to achieved characteristic strength on the 28th day.